WASHINGTON — In the office of Attorney General Jeff Sessions hangs a portrait of his predecessor Edwin Meese III, the Reagan-era conservative. Near the desk of the deputy attorney general is a painting of former Attorney General John Ashcroft, who pressed to expand the Justice Department’s powers after Sept. 11.

The department’s inspector general, Michael E. Horowitz, chose for his office a portrait of the disgraced Attorney General Harry M. Daugherty, who was nearly impeached, forced to resign in 1924 and later tried twice on charges of defrauding the government.

Mr. Daugherty’s visage serves as a reminder for Mr. Horowitz: Malfeasance is never far away.

Equal parts auditor, investigator and cop, Mr. Horowitz has navigated his role as one of the most powerful arbiters of conflict in Washington — the investigators’ investigator — with a diplomat’s instinct for recognizing fault lines, a prosecutor’s focus on justice and a Washington insider’s knack for amassing allies on Capitol Hill.

Now, he has been thrust into the clash of law enforcement and politics that has consumed much of the capital for the past two years. On Thursday, he will issue the highly anticipated findings of his examination of the F.B.I.’s handling of its investigation into Hillary Clinton’s use of a private email server. He is expected to castigate the decision making by the former F.B.I. director James B. Comey; his deputy, Andrew G. McCabe; and former Attorney General Loretta Lynch.

Another high-profile test looms beyond that. Mr. Horowitz, who declined to be interviewed for this article, has begun a review of aspects of the Russia investigation. His findings could land in 2020 amid the presidential race.

In interviews with nearly two dozen friends, former colleagues and political leaders, a portrait emerged of Mr. Horowitz as a principled, savvy investigator. Mr. Horowitz is good-humored and even-tempered, said past co-workers, who ribbed him for his cameos as a prosecutor on “K Street,” HBO’s short-lived 2003 show about lobbyists. He delivered lines like, “You need to have at least a general understanding of the financial flow of funds,” with the élan of a career civil servant.

He has faced criticism over his most highly charged reports, including on the Fast and Furious gunrunning scandal. In it, he offered harsh assessments about one official, but critics expressed concern that the report inflated the official’s role in the episode.

His allies said that attacks were inevitable. In the Fast and Furious report, “I disagreed with some of Michael’s conclusions, but not his motives,” said John Roth, a former federal prosecutor and former inspector general of the Department of Homeland Security. “You don’t take this job to make a lot of friends.”

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“You don’t take this job to make a lot of friends,” another former inspector general said of Mr. Horowitz.CreditJ. Scott Applewhite/Associated Press

Mr. Horowitz, 55, grew up in Suffern, N.Y., his mother a purveyor of antiques and his father the owner of the women’s clothing maker Paul Allen. He attended Brandeis University and Harvard Law School and later married Alexandra Kauffman, then a field producer at CNN.

In 1991, Mr. Horowitz joined the federal prosecutor’s office in Manhattan, where he briefly overlapped with Mr. Comey. He led a huge investigation that implicated almost an entire police precinct in Harlem in a scandal involving drug dealing, robbery and bribery.

“It took an extraordinary amount of independence,” said Mary Jo White, the former United States attorney in Manhattan. “Prosecutors work with police on their cases, so he was investigating and prosecuting, in effect, his partners in law enforcement.” He won an award from the attorney general, and Ms. White named him head of her office’s public corruption section.

In 1999, Mr. Horowitz moved to Washington to join the Justice Department’s criminal division and became chief of staff to its leader, James K. Robinson, where his judgment was lauded. “He had to know what to ask the prosecutors, without having intimate knowledge of a case, judge the quality of the incoming information and send it to the boss,” said Shan Wu, who was counsel to Attorney General Janet Reno, noting that Mr. Horowitz took notes on a single notecard and simply remembered the rest of the information.

Mr. Horowitz left the Justice Department in 2002 to join Cadwalader, Wickersham & Taft as a defense lawyer and was later nominated by President George W. Bush to the United States Sentencing Commission. In 2012, President Barack Obama tapped him to be the Justice Department’s inspector general.

By the time Mr. Horowitz started his job, his office was well into its investigation of the Fast and Furious scandal, a botched operation in which officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives let guns enter Mexico in the hopes of tracing them back to criminals.

Mr. Horowitz handled the politically fraught situation by reaching out to Democrats and Republicans on the congressional oversight committees. In a meeting with former Representative Jason Chaffetz, Republican of Utah, and Representative Trey Gowdy, Republican of South Carolina, Mr. Horowitz told them about his own conflicts of interest and said he had assigned others in his office to handle those parts of the investigation.

“It went a long way to help bolster our confidence that he’d call balls and strikes fairly,” Mr. Chaffetz said, adding that no inspector general had taken that step before with him.

Mr. Horowitz’s eventual report faulted law enforcement officials for poor communication and a failure to see red flags. He assigned significant blame to Jason Weinstein, the deputy assistant attorney general of the Criminal Division. Mr. Horowitz was critiqued for focusing too much on a career prosecutor in Washington, rather than the A.T.F. leaders in Arizona, the center of the operation, or the department’s top political appointees.

Mr. Weinstein accused Mr. Horowitz of using him as a scapegoat.

“I have been singled out because of the desire to blame someone of rank within Main Justice, even though my only knowledge about Fast and Furious consisted of repeated false assurances from those who supervised the investigation,” Mr. Weinstein said in a statement at the time.

Others defended Mr. Horowitz’s report. It “was highly critical of the department and was accepted by both sides of the aisle,” said Representative Elijah E. Cummings of Maryland, the leading Democrat on the oversight committee.

Mr. Horowitz also leveraged his tactical know-how amid an internecine battle with the F.B.I. over documents and other information that his office viewed as essential to perform oversight. The bureau had refused requests to turn over the material.

He became a key behind-the-scenes player in the 2016 passage of a law intended to reinforce the power of inspectors general, working with colleagues to lobby all 72 government watchdogs at the time to sign a letter to lawmakers outlining their challenges. Two years earlier, only 47 had signed a similar letter.

“An access issue for one I.G. is an access issue for all of us,” said Allison C. Lerner, the inspector general at the National Science Foundation. “Michael thought that we had to speak out as a community on that fundamental issue, and many of us agreed.”

Mr. Horowitz ensured the backing of his fellow inspectors general again after the 2016 election by helping protect them from political interference, in large part through his relationships with lawmakers.

At the time, Trump transition officials had told some inspectors general they would be replaced. But under longstanding norms, they are treated more like judges, who decide when to leave. Mr. Horowitz immediately reached out to allies on Capitol Hill; no one was let go.

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Mr. Horowitz, right, has leveraged his relationships on Capitol Hill several times during his six years as the Justice Department’s inspector general.CreditJacquelyn Martin/Associated Press

“Congress understands that I.G.’s are often their main source of desperately needed information on routine and significant threats to the agencies,” said Paul C. Light, a New York University professor who studies the offices.

Now, Mr. Trump poses a unique threat to Mr. Horowitz and his credibility as a neutral judge.

Responding to Justice Department officials and Congress, Mr. Horowitz initiated in March a review of the F.B.I.’s surveillance of Carter Page, the former Trump campaign official. But Mr. Trump’s proclamations that the bureau had abused its power contributed to a perception that the Justice Department was conceding to his wishes, rather than acting independently.

Still, law enforcement officials may have put some distance from the president’s order by referring it to Mr. Horowitz.

“No one orders inspectors general to do things,” Mr. Light said. “They determine the agenda. They take input and decide whether to accept the request.”

Mr. Horowitz has already issued a short but scathing report that found Mr. McCabe had lacked candor about his dealings with a journalist. Mr. Trump crowed about the findings as he had when Mr. McCabe was fired hours before he was eligible for retirement. “He LIED! LIED! LIED!,” Mr. Trump said on Twitter, adding, “McCabe is Comey!!”

"No one, not even an independent inspector general, is fully immune from the type of political pressure that has been applied in Mr. McCabe’s case,” Michael R. Bromwich, a former inspector general who represents both Mr. McCabe and Mr. Weinstein, wrote in a statement.

Mr. Trump is already laying the groundwork to attack Mr. Horowitz’s coming report. “What is taking so long with the Inspector General’s Report on Crooked Hillary and Slippery James Comey,” he wrote on Twitter this month. “Hope Report is not being changed and made weaker!”

But Mr. Horowitz’s alliances, cultivated over years in Washington, may serve to pad him from the president’s criticism. Mr. Gowdy, who as the chairman of the House oversight committee has been one of the Justice Department’s sharpest critics, called him “professional, fair, fact-centric and evenhanded.”

Added Mr. Chaffetz: “Republicans and Democrats alike really need to take to heart whatever it is he finds, even if it doesn’t fit our political agendas. People will spin and throw political barbs, but if you read the actual text, my gut is that’s the truth.”